Adhere, join, stick to and follow the opponent, without letting go or resisting
It is said, “Lure the opponent’s advance into emptiness; harmonize with him, then issue power. Adhere, join, stick to and follow the opponent, without letting go or resisting,” [that is, follow the opponent on both the vertical and horizontal planes]

Follow the opponent’s incoming posture and lead him into emptiness. As I lead him in, I issue my own attack. The word “lead” actually has two meanings. The first is to accord with the opponent’s posture and draw him further in order to take advantage [of his momentum]. The second is to feign weakness, causing him to rush in brashly. We read in Chen Xin’s Boxing Treatise, “Entice the opponent with an ‘empty basket’; then just make one turn.” Enticing with an empty basket is the same as “Lure the opponent’s advance into emptiness.” ” Turning” means striking the opponent.

The older generation says, “People who practice push-hands live according to the principle of ‘neither let go nor resist’.” Not letting go means not quitting the opponent’s hand. Not resisting means not opposing him. This concept includes adhering and joining on a vertical plane, as well as horizontal sticking and following. Adhering motions belong to the category of “not letting go”. Following and joining motions belong to the category of “not resisting”. That is to say, when the opponent advances, I follow and join his motion. And if he retreats I adhere to him.
(from a Study of Taiji Push-Hands by Xiang Kairen)